Nutrition is a basic requirement for life and plays an important role in health and in disease prevention, but malnutrition is a common event and a cause of increased morbidity and mortality, particularly in patients with disease-related malnutrition showing inflammation and a catabolic state. Malnutrition is often overlooked, and deterioration in the nutritional status following admission to hospital is common. It should be actively pursued by a ubiquitous system of nutrition screening, and full nutritional assessment is required for those found to be at risk. There are simple screening tools which can be used by all health care professionals. Assessment considers body composition, inflammatory status and other aspects of underlying diseases and their functional consequences; it is a more specialist process. It is important to determine the energy and protein needs of each individual patient. Appropriate nutritional intervention can often be offered by the oral route, using food with or without special supplements. When this is insufficient, enteral tube feeding will normally be sufficient, but there is an important subgroup of patients in whom enteral feeding is contraindicated or unsuccessful, and in these patients parenteral nutrition (either total or supplemental) is required. A number of immunonutrients and other special substrates have been shown to be helpful in specific circumstances, but their use is not without potential hazards, and therefore adherence to international guidelines is recommended.